This invention relates to a heat pressure-fixable capsulated toner, and an electrophotographic imaging apparatus and toner used therein.
Conventional electrophotographic printers form an electrostatic latent image by first electrifying a photoconductive insulating layer uniformly, then subjecting the layer to exposure, and dissipating the charge on the exposed portion to form the electrostatic latent image. Furthermore, it requires a developing step of depositing a fine charged powder (referred to as a toner hereinafter) onto the latent image to form a toner image, and a transferring step of transferring the resultant toner image onto paper, and a fixing step of fixing the image by heating, pressure or other suitable fixation procedure.
In general, a toner is subjected to a variety of mechanical stresses caused by rolling action of a developing roller and a toner-supplying roller during operation in a developing device, and mechanically deteriorates over a long period of operation. It has been known that the use of resins having a large molecular weight is generally useful to prevent the deterioration of the toner. However, it is necessary to heat a heat-roller to a high temperature for the satisfactory fixation of the resins onto paper. This causes some problems, such as a need for enlargement of the fixation device, curling of the paper, large energy consumption and deterioration of the fixation device.
In order to solve the problems mentioned above, a proposal has been to use a capsulated toner consisting of a core and a shelf which covers the surface of the core. This proposal intends to obtain a capsulated toner having both good fixability and good blocking resistance by a combination of making the core of easily fixable and thermally soft materials, and the outer shell made of comparatively hard material having a good blocking resistance.
Some proposals have been made on a variety of techniques using a wax having a low melting temperature such as a liquid wax and a rubber-like wax at room temperature, as a core material. Generally a hard material is selected as a shell material. When the shell material is soft and has low strength, although the fixation is improved, it would be difficult to obtain the intended toner property since the shell material would separate from the core material and the toner itself would deform due to stress caused by a developing device. When the shell material is hard and strong, a large pressure and a high temperature are required to collapse the shell, causing a bad fixation itself.
Then, a proposal has been made to use as the core material, resins having a low glass transition temperature (which will be referred to as a Tg hereinafter), which causes a blocking at a high temperature when it is used alone but improves fixation strength, and as the shell material to use a resin layer having a high Tg in order to give blocking resistance i.e. a proposal for a capsulated toner for heat-roller fixation. Usage of such toner is one effective means to achieve a fast and small apparatus with low electric power consumption.
The prior art toners above-mentioned have problems to be solved as follows.
In order to obtain a balance of good fixability and high-temperature stability in a capsulated toner, incompatibility between the shell resin and the core resin is essential. Therefore, it is necessary to use a resin for the shell that has lower electric resistance than the core, and also has a higher polarity structure than the core resin. More specifically, a capsulated toner having the advantage of stability and low-temperature fixability requires using for the shell a resin with relatively higher polarity structure than the core. However, if a resin with a high polarity structure is used, charge is easily dissipated from the surface of the charged toner particle so that it becomes disadvantage for the developing and transferring treatments. As a consequent, it is one factor that makes the design of the charge property of the toner difficult.